This invention relates to motor controllers and, more particularly, to a motor controller that automatically compensates sensed current to reduce the errors associated with sensing average motor current.
Solid state starter/controllers have found widespread use for controlling application of power to an AC induction motor. One type of starter/controller is a reduced voltage solid state starter (RVSS). The starter/controller, referred to hereinafter as simply a controller, uses SCRs, or triacs, for controlling application of AC line voltage to the motor. The SCRs are fired once during each half cycle of line voltage for controlling application of AC line voltage to the motor. The SCRs turn off at zero current.
A typical controller, such as for an elevator, is calibrated to read RMS currents from 5% to 450% of the controller""s rating with pure sinusoidal AC waveforms. A sinusoidal current is used during calibration. As the current during calibration is sinusoidal, a constant conversion factor causes a sensed average motor current developed by a filter to match the actual RMS current flowing through current transformers. However, during reduced voltage starting, the SCRs are fired for only a portion of the cycle of line voltage. As a result, the AC current to the motor is actually non-sinusoidal during starting. Also, during overload conditions, if the current the motor draws exceeds the settings for the starting current limit, then the controller phases back the voltage to limit the current to the starting current limit setting. This also results in a non-sinusoidal current being sensed by the current sensors. As the incoming current waveform becomes more non-sinusoidal, the difference between average motor current given by the filter and the true RMS current increases. Depending on the setting for the overload devices and starting current limit, the error can be significant and cause large differences between the current the unit calculates and the actual RMS current. In overload conditions where the voltage to the motor is reduced to limit the current to the starting current value resulting in a discontinuous AC current waveform, the overload tripping times can be significantly longer when compared to a continuous AC waveform with the same RMS value.
Typical softstarter controllers do not compensate for the above error. This causes the current during start to exceed the setting for the maximum starting current. Each controller is rated for several different sized motors. The smaller the motor, the more sinusoidal the current for a particular setting. On the largest motor the controller is rated for the error which could be as large as 15-20%.
In accordance with the invention, there is provided a motor controller which compensates sensed motor current to reduce errors associated with sensing average motor current.
Broadly, there is disclosed herein a motor controller for a motor operated by an AC line voltage including solid state switch means connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor. A current sensor senses motor current. A control circuit is connected to the switch means and the current sensor for varying relative duration of on time and off time during each cycle of line voltage to control motor voltage. The control circuit includes means for determining on time during each cycle of line voltage and compensating sensed motor current responsive to the sensed on time.
It is a feature of the invention that the solid state switch means comprises a silicon controlled rectifier or a triac.
It is another feature of the invention that the determining means comprises a sensing circuit for sensing voltage between the AC line and the motor terminal.
It is another feature of the invention that the control circuit comprises a programmed microprocessor and a memory storing a table correlating on time to a compensation factor and the microprocessor modifies the sensed current based on the compensation factor. The microprocessor may be programmed to interpolate the compensation factor between stored on time values in the table.
It is another feature of the invention that the current sensor comprises a current transformer and a conditioning circuit for averaging sensed motor current.
There is disclosed in accordance with another aspect of the invention a motor controller for a motor operated by an AC line comprising solid state switch means connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor. A current sensor senses motor current. A voltage sensor senses voltage across the solid state switch means. A control circuit is connected to the switch means, the current sensor and the voltage sensor for varying relative duration of on time and off time during each cycle of line voltage to control motor voltage. The control circuit includes means for determining on time during each cycle of line voltage responsive to the voltage across the solid state switch means and compensating sensed motor current responsive to the sensed on time.
There is disclosed in accordance with yet another aspect of the invention a motor controller for a motor operated by an AC line voltage comprising solid state switch means connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor. A current sensor senses average motor current. Time sensing means determine on time of the solid state switch means during each cycle of line voltage. Current compensation means compensates sensed average motor current responsive to the sensed on time to determine RMS motor current.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.